1. Field of the Invention
The invention pertains to the field of aircraft landing aid systems and more particularly to a distance estimator for a microwave landing system.
2. Description of the Prior Art
The Microwave Landing System (MLS) has two scanning beams, one in azimuth and one in elevation. Signals from these beams are received separately in a receiver/processor which calculates the elevation angle relative to the ground and the azimuth angle relative to a glide slope beam. These angles, distance to go information from Distance Measuring Equipment (DME), and auxiliary data such as minimum glide path, are utilized to guide an aircraft during a landing approach. Glide slope tracking accuracy is a function of the distance to go and the accuracy to which this distance is known. Normally, the distance to go is accurately provided by the DME. Without the DME, poor performance generally occurs, especially if the radio altitude signal is saturated to its full scale value, that is, above 2,500 feet. Control law designs of the prior art have incorporated procedures to weight the best performance to those maneuvers which have the highest probability of occurrence. These designs, however, exhibit poor stability for many maneuvers. To provide better stability, performance must be sacrificed. Such compromises between performance and stability are unacceptable for the MLS aircraft landing control system.
It is an object of the present invention to furnish a distance estimator when the DME is not available, which provides high performance, stable operation.